2-Acetyl-6-methylnaphthalene (2,6-AMN) and 2-acetyl-6,7-dimethylnaphthalene (2,6,7-ADMN) are useful feedstocks for the production of 2,6-naphthalene dicarboxylic acid and 2,3,6-naphthalene tricarboxylic acid, respectively, by oxidation. Such acids are useful monomers for polymerization into high performance resins (or so-called engineering resins).
2,6-AMN and 2,6,7-ADMN can be prepared by various synthetic routes. One promising route involves Friedel-Crafts acylation of the corresponding 2-methylnaphthalene (2-MN) and 2,3-dimethylnaphthalene (2,3-DMN). The same tri-acid can be obtained by oxidation of 2-acetyl-3,7-dimethylnaphthalene, obtained by acylation of 2,6-dimethylnaphthalene (2,6-DMN), but the oxidation is believed to be undesirably sluggish.
Preparation of acyl aromatic ketones using Lewis acid metallohalide catalysts has been research subject matter since the original discoveries of Friedel and Crafts reported in 1877. The use of a single solvent and complexing agent such as nitrobenzene to promote beta position substitution in Friedel-Crafts acylations of naphthalene ring systems has been known for many years. A synthesis of 2,6-AMN by condensing acetyl chloride with 2-MN in nitrobenzene is believed to have been first reported by Kon and Weller, J. Chem. Soc., 1939:792. The reaction is illustrated by the following equation: ##STR1## However, the reported isolated yield of 2,6-AMN was only 33 percent of theoretical. Since a 70 weight percent yield of the mixed isomeric ketones was isolated from the crude reaction mixture (by vacuum distillation), regiospecificity to the 2,6 isomer was characteristically undesirably low, probably on the order of about 50 percent.
A further disadvantage of this synthesis is that it requires the use of relatively large amounts of nitrobenzene which is undesirable particularly from a commercial processing standpoint because of the great toxicity of nitrobenzene. This material is categorized as a harzardous waste substance by the EPA. Thus, process waste water would have to be almost completely free of residual nitrobenzene to comply with EPA regulations.
An acylation procedure for 2-MN and for 2,3-DMN which would have high regiospecificity for producing beta position acylation and particularly the acetylation of the naphthalene nucleus, and which avoids the use of large amounts of nitrobenzene, would have value as a commercially practical procedure for making 2,6-AMN and 2,6,7-ADMN. The present invention provides such a procedure.